I am not familiar with either Zion or Activity Pub, but I don't think the idea is stupid.
if you don't know what it is, how can you think it is not stupid?
Besides, there isn't just one internet browser, there are many different types. There isn't one document creating and editing software, there are multiple.
social networks are like the internet a network, same with the telephone for example or language, or money (like bitcoin). there will be only one in the longterm (or at least one network which dominates), there will probably always be people who don't understand what networks are and therefore other monies, other internets, other social networks will exist, but as a minority...
Zion claims that content creators can get paid for their creations and those who consume the content can get in on it as well. Compare that with traditional social media platforms, you get nothing unless you are a famous person with millions of followers. On the other hand, the platforms make money on your data and interests.
wow such a great claim, did you know that you can get paid right now for your creation? as i said, the idea is stupid